BRASS: We build mirrors 8.4 meters or 6.5, and our first step is to put down silicon carbide floor plates.

Beneath Arizona Stadium in Tucson, four giant slabs of glass are taking shape — the mirrors of future telescopes. Placed on edge, each mirror is taller than a two-story building, and its surface is as roomy as a small apartment. Yet each is relatively thin and lightweight.

The mirrors are produced by the Steward Mirror Laboratory at the University of Arizona. They’re the largest single-glass telescope mirrors yet built. They start with 20 tons of raw glass that’s melted inside a spinning oven. From inside the busy lab, Alan Brass explains:

BRASS: We've got 20 tons of glass loaded in, put the cover to the furnace on, start it spinning, and it takes about a week to get up to temperature. At 2130 degrees the glass fully is the consistency of honey...

As it spins, the glass forms the curvature needed to bring starlight to a sharp focus. After the mirror is cast, it cools for several months. Then its surface is ground and polished to a precise shape. After that, it’s tested to make sure it has the right shape — a flaw of a thousandth of the width of a human hair would spoil the view.

Finally, it’s packed and shipped to its destination — an astronomical observatory. After final assembly and testing, it’ll get to work — uncovering the secrets of the universe.